1. Field of the Invention
The invention relates generally to restorative dental technology, and in particular to method and apparatus for preparing a wax replica of a dental restoration.
2. Description of the Prior Art
Modern dental practice involves a variety of casting operations, ranging from the simplist inlay to all forms of cast crowns, bridge structures and removable partial dentures, each of which makes use of the same fundamental practices in forming the cast restoration. The structures produced by casting operations represent an important part of restorative dentistry.
In dentistry, the casting procedure is known as the "lost wax technic". Basically, this method consists of forming a wax pattern, surrounding it with investment material, and later heating the investment mold to remove the wax before casting melted metal into the mold. In all instances, regardless of the technic employed or the type of restoration to be cast, it is necessary first to prepare in wax an exact duplicate of the finished restoration. When the formation of an acceptable wax pattern is accomplished, it is surrounded with dental investment, after which investment is permitted to harden to form the mold into which a dental alloy is cast.
When the investment mold has been formed and the wax eliminated by heating, it is then ready to receive the molten metal to form the casting. Proper and careful heating of the investment mold is necessary in order to provide complete wax elimination from the mold. All conventional investment materials display some setting expansion and hygroscopic expansion upon hardening and thermal expansion when they are heated to eliminate the wax from the mold or when they are heated to more elevated temperatures associated with the casting procedure. Because the purpose of the metal casting is to provide a metallic duplication of missing tooth structure with as great an accuracy as possible, it is essential to minimize the expansion associated with investment casing.
The factor of expansion in addition to other factors such as flow and warpage is important in the preparation of a wax pattern which will serve to produce an accurate gold casting. The two most commonplace methods of preparing a wax pattern are the "direct" and "indirect" technics. The method of preparing a wax pattern is called "direct" if the wax pattern is prepared in the patient's mouth. On the other hand, if an impression is taken from the cavity preparation and a die is formed from the impression on which the pattern is prepared, then the method is called "indirect".
When a wax pattern is formed by the indirect method, a dental stone die is used which is the positive replica of at least a portion of the surrounding tooth structure and the cavity preparation. This tooth replica permits the pattern to be formed outside of the mouth. A cavity for receiving a gold crown is typically prepared by cutting away surrounding portions of the tooth to form horizontal and vertical surfaces. Because of this special cavity preparation, the lateral dimensions of the gold crown can be adjusted relatively easily in the laboratory to insure a good fit. However, vertical expansion of the gold crown is extremely difficult to correct because of the complex shape of the occlusal surface. According to conventional corrective procedures, correction is carried out by grinding and polishing after the crown has been secured to the prepared cavity in the patient's mouth. A good occlusion fit is very difficult to obtain by this procedure because of the complex contour of the occlusion surface and because of limited access when working in the patient's mouth. Therefore it would be desirable to provide method and apparatus for compensating for the thermal expansion of the gold casting and thereby minimize the amount of "touch up" dental work required to provide a good occlusal fit.